Fabrication of thin films has generally used a stream of chemical components directed towards a substrate for deposition thereon. The process takes place in a chamber filled with a process gas under a specific pressure or under vacuum conditions. The chemical components involved in the formation of the thin films may be in the form of atoms, ions, molecules, radicals, and/or other small size clusters of the material elements to be deposited on the surface.
Ablation is a technique employed to generate a stream of chemical components by exposing a limited volume of the target material to energy impact where the level exceeds the bonding energy of atoms in the material volume. This increases the temperature of the exposed volume to a level which exceeds the vaporization temperature of any element of the material volume. The material is converted into a dense, high temperature, and high-pressure cloud of plasma which is ejected from the surface of the target and forms a film deposition stream. The non-thermal equilibrium character of ablation provides for its distinguishing features, e.g., the ability even for multi-component materials to generate the stream of plasma with a composition close to that in the ablation volume.
A high power density (≧108W/cm2) flux of energy is required to load the target volume with energy sufficient for material ablation. Usually, pulsed generator devices such as lasers or electron beam generators are used for this purpose. When a pulsed laser beam is used as the energy source for ablation, the film formation process is known as Pulsed Laser Deposition (PLD). When the pulsed power is the flux of energetic electrons used for ablation, then the plasma stream generation process is known as Pulsed Electron Ablation (PEA) and the film deposition technique is Pulsed Electron Deposition (PED). The use of fast electrons rather than eximer laser radiation (as in conventional laser ablation methods) enables ablation of materials that do not couple well to the laser due to either their partial transparency at the laser wavelength, or reflectivity considerations.
To enable PED for complex materials over a wide range, PEA apparatus is needed which has the capability of providing high power density flux of electrons. For example, for a target material with a large thermal conductivity, a higher flux density than for metal oxide target is required. Higher power density compensates for the energy losses from the ablation volume. Higher power density also results in a more energetic plasma stream that is advantageous for formation of some films. The source of the electron beam in PEA and PED processes is of great importance.
An electron flux (beam) source based on what is commonly referred to as “channel-spark discharge” was introduced by C. Schultheiss in U.S. Pat. No. 5,576,593. The device, shown in FIG. 1, is a linear electron accelerator comprising a periodic structure of ring electrodes (auxiliary anodes) 10 separated by segments of dielectric tube 12. At one end, the tube 12 is connected to a metallic reservoir (cathode) wall 14 maintained at a negative potential in which a pulsed plasma 16 is created by a trigger (not shown). The plasma 16 serves as the source of electrons drawn into the tube 12 under the action of an electric field between the cathode 14 and the anode 10. Isolator 18 separates the cathode 14 and the anode 10. The role of the accelerator tube assembly is two-fold. First, it prevents the self-breakdown of the device that is spontaneous (without triggering) discharge of a capacitor 20. The second, it guides the flux of electrons (beam) emitted by the plasma 16 in the cathode reservoir 14 along the tube 12. Driven by the potential difference between the cathode plasma and the anode, the electrons reach an energy level of up to 10-20 keV as defined by the applied voltage between the cathode 14 and anode 10. At certain conditions, when gas pressure in the process chamber 22 is of an optimum level of ˜0.2 Pa, it is possible to generate a well-collimated beam 24 directed by the tube 12, which emerges from its exit 26, and is able to propagate beyond the tube 12 due to the beam-induced space charge neutralization.
For the purpose of materials ablation, the electron beam power density, as well as the electron energy is of primary importance. Since the power density is proportional to the product of the beam current density and the source voltage, it is the electron current density that controls the beam power at a given operational voltage. However, since the boundary of the plasma is the source of the electron beam, the beam current and the power of the electron beams are limited in part by the ability of plasma in the plasma source to supply electrons to its boundary, e.g. are limited by the concentration of the plasma.
The device shown in FIG. 1 is thus somewhat deficient in its ability to deliver the electron beams of superior electron current density. An electron beam source capable of generating a high electron beam power density to be delivered to the surface of a target is needed to enable PEA and PED processes for a wide range of materials.